Conventional double heterostructure diode lasers generally lase at several longitudinal modes simultaneously. Since each longitudinal mode is at a different wavelength this type of laser is not as useful as it might be if its output could be restricted to a single dominant longitudinal mode. Specifically, the applications requiring coherence, for example holography, could be carried out with a single mode laser but not as effectively with one that emits multiple modes. In addition, single mode optical fibers are best served by an optical source having a single dominant output wavelength.
One technique for achieving longitudinal mode control in a double heterostructure laser is set forth in U.S. Pat. No. 4,132,960 issued Jan. 2, 1979, to W. Streifer, et al. In this patent the active region of a double heterostructure semiconductor laser is heavily doped with a dopant such as zinc or germanium with a density of approximately 10.sup.19 /cm.sup.3. This technique of using a heavy doping intensity of the active region has been found to achieve longitudinal mode control in lasers that are operating at high pulse powers. The high density dopants essentially prevent hole burning in the carrier density or gain. In low powered continuous semiconductor lasers having outputs in the order of 1 to 10 .mu.watts this technique of high density dopants is not effective in achieving longitudinal mode control.
Longitudinal mode control in a double heterostructure semiconductor laser has recently been observed by N. Nakamura, et al as reported in their article entitled "Longitudinal-mode behaviors of mode-stabilized Al.sub.x Ga.sub.1-x As injection lasers" Applied Physics, Vol. 49, No. 9, September 1978, pages 4644-4648. The laser reported in this Nakamura et al article was said to lock onto one mode and remain in that mode even though temperature variations of several degrees were encountered. The article attributes this behavior to some type of "reduction of gain around a lasing mode" based on a nonpublished analysis by M. Yamada and Y. Suematsu of Japan. There is no disclosure in the article as to how to construct a laser to duplicate these results.